Subsea Power Research Articles

Cavities in mass-impregnated HVDC subsea cables studied by AC partial discharge measurements

Partial discharge measurements have been used for studying shrinkage voids in the insulation of mass-impregnated high voltage DC subsea power cables. Three 4.5-m long cable samples were subjected to ac partial discharge measurements at different ambient temperatures, both under isothermal conditions and after load current turn­offs. Two distinctly different phase resolved partial discharge patterns were observed, suggesting that two different types of cavities were created. The type causing by far the most powerful discharges appeared a few hours into the cooling period after load turn-off, which coincides in time with when load cycling breakdowns during type testing usually occur. Current loading has a significant effect on the cavities, as it leaves them — at least temporary — with a different dielectric strength afterwards. The mechanisms behind this change are not identified, but the associated time constants are of the order of days and weeks. Consequently, accurately determining the limitations of such cables with regard to their ability to withstand polarity reversals and rapid load changes may be more complicated than previously assumed.

Cavities in mass-impregnated HVDC subsea cables studied by AC partial discharge measurements

Partial discharge measurements have been used for studying shrinkage voids in the insulation of mass-impregnated high voltage DC subsea power cables. Three 4.5-m long cable samples were subjected to ac partial discharge measurements at different ambient temperatures, both under isothermal conditions and after load current turn­offs. Two distinctly different phase resolved partial discharge patterns were observed, suggesting that two different "types" of cavities were created. The type causing by far the most powerful discharges appeared a few hours into the cooling period after load turn-off, which coincides in time with when load cycling breakdowns during type testing usually occur. Current loading has a significant effect on the cavities, as it leaves them — at least temporary — with a different dielectric strength afterwards. The mechanisms behind this change are not identified, but the associated time constants are of the order of days and weeks. Consequently, accurately determining the limitations of such cables with regard to their ability to withstand polarity reversals and rapid load changes may be more complicated than previously assumed.

Strain controlled medium cycle fatigue of a notched Pb-Sn-Cd lead alloy

During the extrusion process of subsea power cable sheathing layer it is possible that metallic and/or non-metallic debris present in the processing environment enter the metal lattice originating discontinuities that might have a detrimental effect on the fatigue life and the overall integrity of the sheathing. In order to understand the influence of these production defects on the reliability of installed power-lines, a series of specimens directly retrieved from the extruded sheathing were fatigue tested at different strain rates and range both in presence and absence of a non-passing through notch simulating the geometrical discontinuity induced by a particle. In order to collect the necessary information for the understanding of the failure mechanism, Digital Image Correlation and Scanning Electron Microscopy were used to understand the influence of the testing condition on the material resistance and failure mode.

A methodology for design and fatigue analysis of power cables for wave energy converters

The recent development of subsea power cables for various offshore marine renewable energy technologies has identified the need for new cables that have low structural stiffness properties. This type of cable is referred to as dynamic cable because of its high bending flexibility compared to static cables. The current study presents a cable design model and simulation models that were developed for the design and fatigue analysis of dynamic cables. These models were applied on a subsea dynamic power cable with a design that is suitable for a floating point-absorbing wave energy converter (WEC), where the cable must withstand cyclic loads imposed by the motions of the WEC, the waves and the ocean currents. The cable design model is presented with its detailed design and dimensioning methodology for cables with multiorder helical structures, with respect to desired (target) mechanical properties. The cable design model is verified against a verification study in the literature. A simulation model of a fatigue test rig for accelerated rotational bending is presented. The results from the numerical simulations and the subsequent fatigue analyses are compared against results from experiments using the test rig. The influence of the dynamic effects and mechanical properties on the fatigue life of the cable is discussed. This study contributes to a better understanding of the fatigue failure mechanisms of the cable, and it also highlights the importance of further development of numerical models.

Predicting Damage and Life Expectancy of Subsea Power Cables in Offshore Renewable Energy Applications

Subsea power cables are critical assets within the distribution and transmission infrastructure of electrical networks. Over the past two decades, the size of investments in subsea power cable installation projects has been growing significantly. However, the analysis of historical failure data shows that the present state-of-the-art monitoring technologies do not detect about 70% of the failure modes in subsea power cables. This paper presents a modeling methodology for predicting damage along the length of subsea cables due to environmental conditions (e.g., seabed roughness and tidal flows) which result in the loss of the protective layers on the cable due to corrosion and abrasion (accounting for over 40% of subsea cable failures). For a defined cable layout on different seabed conditions and tidal current inputs, the model calculates the cable movement by taking into account the scouring effect and then it predicts the rate at which the material is lost due to corrosion and abrasion. Our approach integrates accelerated aging data using a Taber test which provides abrasion wear coefficients for the cable materials. The models have been embedded into a software tool that predicts the life expectancy of the cable and demonstrated for narrow conditions, where the tidal flow is unidirectional and perpendicular to the power cable. The paper also provides discussion on how the developed models can be used with other condition monitoring data sets in a prognostics framework.

Room temperature creep mechanism of a Pb-Sn-Sb lead alloy

Lead alloys are the most common materials adopted for the production of subsea power cable sheathing. The sheathing is a layer of stable and watertight metal, which serves to prevent the electrical failure of the cable. During the predicted operational life of the cables of several decades, these experience strains due to the installation process, the oceanic currents and the thermal expansion of the cable. The low melting temperature of such alloys, around 600 K, imply that creep deformation will occur when subjected to loading even at room temperature. The goal of the present study is to investigate the tensile behavior of the Pb-Sb-Sn alloy of interest in order to predict the correlation between strain rate and stress level. A mechanical characterization was performed through tensile testing at different strain rates of specimens cut from power cable sheathing. Due to the extreme ductility of the material, the use of digital image correlation was necessary to compute an acceptable approximation of the in-plane strain field on the surface of the specimens. The results were implemented in finite element method environment using Abaqus and Isight to calibrate a creep model able to reproduce at best the behavior of the material. Such model was also positively tested in the case of a relaxation test. In addition, a tensile test of several steps at different loads was executed with the aim of extrapolating and interpreting the steady state creep exponents at different creep regimes and the indications that these can provide on the deformation mechanisms of the alloy.

A testing platform for subsea power cable deployment

The failure of subsea cables has been predominantly attributed to third party activities and cable field joints. Field joints may be unavoidable due to joining of insufficient factory-made cable lengths, cable repair, or when the cable installation process has to be abandoned due to rough weather or other unplanned events. Failures within the jointing location reveal that current quality testing of these joints may be inadequate. The failure mode in these joints is mainly a result of seawater ingress at the field joint. Current design practices for subsea cable field joints recommend offshore simulations/trials to demonstrate the long term performance of the joint under the expected mechanical loads during cable installation to demonstrate whether the planned field joining practice is satisfactory. Nevertheless, offshore simulations are costly, and therefore a set of standardized onshore testing schemes would be advantageous alternative. This paper presents an onshore testing scheme for subsea power cable deployment. The proposed testing arrangement can be employed to verify the design of the offshore field joint and to prove the functionality of the field joint under the installation loads.

The direct interconnection of the UK and Nordic power market – Impact on social welfare and renewable energy integration

United Kingdom and the Nordic power market have plans to interlink directly through a sub-sea power transmission line in The North Sea. Such power market couplings have complicated implications for the interconnected energy systems and for different agents in the common power market. We analyse this case by modelling the hourly operation of the Nordic-UK power market coupling, considering the local district heating (DH) system in each country as well. According to the results, after the operation of the new interconnection between Norway and the UK (North Sea Link), the overall socio-economic benefits (social welfare) in the region will likely improve by 220–230 million euro per year, without considering the cost of the interconnector itself. The UK-Nordic market coupling enhances the flexibility of the UK power system in wind integration, irrespective of the share of wind in the Nordic countries. However, increasing wind capacity in the UK will diminish the expected economic benefits of the link. The merit order effect of wind integration in the UK will reduce the price gap between UK and Norway, and so the congestion income of the link in many hours a year when the link is congested from Norway towards the UK.

Behavioral responses by migrating juvenile salmonids to a subsea high-voltage DC power cable

Currently, there is large-scale interest in developing marine-based energy sources and extensive subsea power cable networks. Despite growing concern that local perturbations in the magnetic field produced by current passing through these cables may negatively affect electromagnetically sensitive marine species, e.g., disrupted migration; few studies have examined free-living animals. We used acoustic biotelemetry tracking data to examine movement behaviors and migration success of a magneto-sensitive fish, late-fall run Chinook (LFC) salmon (Oncorhynchus tshawytscha), in relation to the energization of a magnetic field-producing subsea power cable, as well as other potentially influential environmental parameters. We analyzed detection records of tagged LFC salmon smolts during their out-migration through the San Francisco Bay before and after the installation of an 85-km high-voltage direct-current transmission cable. Cable energization did not significantly impact the proportion of fish that successfully migrated through the bay or the probability of successful migration. However, after cable energization, higher proportions of fish crossed the cable location and fish were more likely to be detected south of their normal migration route. Transit times through some regions were reduced during cable activity, but other environmental factors were more influential. Resource selection models indicated that proximity to the active cable varied by location: migration paths moved closer to the cable at some locations, but further away at others. Overall, cable activity appeared to have mixed, but limited effects on movements and migration success of smolts. Additional studies are recommended to further investigate impacts of subsea cables on fish migrations, including potential long-term consequences.

Understanding the effects of electromagnetic field emissions from Marine Renewable Energy Devices (MREDs) on the commercially important edible crab, Cancer pagurus (L.)

The effects of simulated electromagnetic fields (EMF), emitted from sub-sea power cables, on the commercially important decapod, edible crab (Cancer pagurus), were assessed. Stress related parameters were measured (l-Lactate, d-Glucose, Haemocyanin and respiration rate) along with behavioural and response parameters (antennular flicking, activity level, attraction/avoidance, shelter preference and time spent resting/roaming) during 24-h periods. Exposure to EMF had no effect on Haemocyanin concentrations, respiration rate, activity level or antennular flicking rate. EMF exposure significantly disrupted haemolymph l-Lactate and d-Glucose natural circadian rhythms. Crabs showed a clear attraction to EMF exposed shelter (69%) compared to control shelter (9%) and significantly reduced their time spent roaming by 21%. Consequently, EMF emitted from Marine Renewable Energy Devices (MREDs) will likely affect edible crabs both behaviourally and physiologically, suggesting that the impact of EMF on crustaceans must be considered when planning MREDs.

Including the calculation of transient electric field in the life estimation of HVDC cables subjected to load cycles

The need for long subsea power links, the advantages of high voltage DC (HVDC) transmission, and the concerns about overhead lines are promoting a booming growth of submarine and land DC cable lines worldwide [1]. In particular the performance of HVDC extruded cable systems has been improved significantly in recent years. The ratings have increased, with bipolar DC extruded cable systems already in service at voltage and power levels up to 320 kV and 1 GW, respectively [2]-[5], and DC extruded cable systems being qualified at voltage ratings up to 525 kV [6], [7].

Damage assessment for submarine photoelectric composite cable under anchor impact

The impact of dropped anchor on submarine photoelectric composite cables may possibly cause electrical faults, i.e. electricity and optical signal transmission failure. In order to study the impact capacity and structural impact failure mechanism, a test setup is designed originally to examine the structural and functional integrity. A detailed finite element model (FEM) is created, considering material nonlinearity and component interaction. A parametric analysis has been performed to predict the deformation of components and impact forces, under different impact velocities and collision directions. Relationships between the armor layer indentation rate and that of internal power and optical units are achieved. The impact deformation of internal entities can be evaluated intuitively by armor layer indentation. The proposed experimental and numerical methods are well correlated, suitable to assess the impact capacity of subsea power cables and assist the protection design of subsea power cables in engineering.

Experimental and numerical investigation of strain distribution of notched lead fatigue test specimen

The work here presented focuses on the test methodology related to effect of stress concentrators in strain controlled structures. Cable sheathing as used in subsea power cables are investigated by cyclic fatigue testing, Digital Image Correlation (DIC) and 3D Finite Element Analysis. Focus is put on the strain distribution in conventional specimen geometries and under the presence of artificial notches. It is evident that standardized fatigue testing provides limited input to the final fatigue life of strain controlled power cable sheathing both accounting for expected and unintended stress concentrators. The limitations can be explained by measured strain distribution inherent in most fatigue test specimens. The use of DIC and 3D FEM provides valuable insight into both the theoretical and practical stress and strain distribution. This can help in understanding and overcoming geometrical test constraints, when compared to the actual component loading mode.

A Study on Passive Cooling in Subsea Power Electronics

This paper proposes a simplified approach to model the thermal behavior of the insulated-gate bipolar transistors (IGBTs) in a subsea power electronic converter. The models are based on empirical relations for natural convection in water, and IGBT datasheet values. The proposed model can be used in the design of subsea converters and in the reliability analysis of their IGBTs. Experimental results are provided to validate the proposed thermal model. Suggestions are made to minimize the net thermal resistance by introducing a high conductivity thermal material as a mounting plate between the IGBT and the cabinet walls. Impact of the mounting plate dimensions and material properties on the junction temperature of the IGBTs is studied. A case study analysis is made on a 100-kVA converter. Results indicate that the thermal spreading resistances in the mounting plate and the cabinet walls contribute significantly to the overall thermal resistance. Spreading resistances can be mitigated by appropriate design measures. Furthermore, it was observed that the passive cooling in water is not as effective as the forced water cooling. However, the low cost, simple design, and higher reliability of passive cooling systems might make them a favorable choice for subsea systems.

Understanding the effects of electromagnetic field emissions from Marine Renewable Energy Devices (MREDs) on the commercially important edible crab, Cancer pagurus (L.)

Abstract The effects of simulated electromagnetic fields (EMF), emitted from sub-sea power cables, on the commercially important decapod, edible crab (Cancer pagurus), were assessed. Stress related parameters were measured ( l -Lactate, d -Glucose, Haemocyanin and respiration rate) along with behavioural and response parameters (antennular flicking, activity level, attraction/avoidance, shelter preference and time spent resting/roaming) during 24-h periods. Exposure to EMF had no effect on Haemocyanin concentrations, respiration rate, activity level or antennular flicking rate. EMF exposure significantly disrupted haemolymph l -Lactate and d -Glucose natural circadian rhythms. Crabs showed a clear attraction to EMF exposed shelter (69%) compared to control shelter (9%) and significantly reduced their time spent roaming by 21%. Consequently, EMF emitted from Marine Renewable Energy Devices (MREDs) will likely affect edible crabs both behaviourally and physiologically, suggesting that the impact of EMF on crustaceans must be considered when planning MREDs.

Energy analysis of a subsea steam Rankine cycle for the subsea power supply

In this paper, a steam Rankine cycle is proposed for the subsea power supply. The turbine, as the key component, is connected to a power shaft to produce the power output. The pressure at the inner and outside of the turbine shaft seal can be balanced. A mathematical model is established, energy and parametric analysis are applied. The results show that this power cycle can acquire a thermal efficiency of 35% at 5000 meters depth. Besides, the electrical heating temperature and the condensing pressure are the key parameters that dominant the performance of this cycle.

Analysis of the influence of ambient conditions on the ampacity of Kevlar-armoured subsea power cables

Analysis of the influence of ambient conditions on the ampacity of Kevlar-armoured subsea power cables

Statistical analysis of dielectric breakdown of liquid insulated printed circuit boards

For introducing power converters at ambient pressure at large sea depths, dielectric liquids are proposed as insulation for power electronic converters. This paper presents statistical analysis of short time tests of partial discharge and breakdown voltage on the trench between metallized edges of printed circuit boards (PCBs) embedded in in different dielectric liquids. Performance of Nytro 10XN, Midel 7131 and Galden HT230 were investigated. Investigations were done both for sinusoidal voltages and for positive fast transient square voltages. Galden as insulating liquid has the highest withstand voltage with higher breakdown voltage at low probability of failure. The overall evaluation shows that all the liquid samples possessed high dielectric strength that has the potential of providing sufficient insulation for subsea power converters. Coating the metal surfaces with thin layer of parylene increases the system reliability by reducing the effect of contaminations.

Temperatures in shallow marine sediments: Influence of thermal properties, seasonal forcing, and man-made heat sources

Analytical and 2D finite-element solutions of the heat transfer equation are used to model the temperature evolution in the upper 15m of marine sediment deposits. These calculations comprise realistic, in-situ measured, thermal properties as well as natural (geothermal and sea bottom temperature) and man-made (e.g. high voltage, subsea power cables) heat sources. A main objective of this article is to focus on the influence of the thermal heterogeneity of sediments in the North and Baltic Seas. In combination with seasonal variations of bottom water temperatures, these result in large temperature differences in the upper 3–5m of the sediments. Also, the impact of additional heat through power cables is considered. The modelling results show maximum cable temperatures as much as 16K difference less compared to a conservative homogeneous scenario previously used from literature values. It can be shown that the 2 K-criterion is not a suitable instrument to monitor the man-made impact to the thermal conditions without knowing the actual thermal settings of a given location.

The great baltic disconnect [News

Former Soviet Union satellites such as Ukraine, Georgia, and the three Baltic states-Lithuania, Latvia, and Estonia-rely heavily on Russian energy, a dependence that colors the ongoing geopolitical tussle among the Russian Federation, the European Union, and NATO. But new energy infrastructure is beginning to free the Baltics from this drama, including two high-voltage direct current links to Lithuania nearing completion: the 500-megawatt LitPol Link with Poland and the 700-MW NordBalt cable to Sweden, which is among the world's longest subsea power links.